Glass-ceramic materials are crystalline or semi-crystalline materials produced from precursor glasses by in situ crystallization. The crystallization process initially requires a glass that contains one or more nucleating agents that permit a controlled crystallization process. A variety of nucleating agents have been disclosed in the prior art. The most commonly used nucleating agent is titania (TiO.sub.2) alone, or in conjunction with zirconia (ZrO.sub.2).
ZrO.sub.2 may be used alone as a nucleating agent for a transparent, beta-quartz solid solution glass-ceramic. This is disclosed, for example, in U.S. Pat. No. 3,252,811 (Beall). However, it is difficult to incorporate ZrO.sub.2 into a glass melt, requiring high melting temperatures and/or long melting times.
The next step is heat treatment of a suitably selected glass in accordance with a controlled time-temperature schedule. Usually, the heat treatment occurs in two stages. In the first stage, crystal nuclei are formed in the glass. This is followed by a second stage during which crystals grow on the nuclei. The heat treatment may occur in conjunction with cooling and forming of a molten glass. Alternatively, a glass article may be formed and cooled, and subsequently heat treated to cause crystallization.
In situ crystallization of glasses in the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 composition field generally provides highly crystallized glass-ceramics. The primary crystal phase, depending on glass composition and heat treatment, may be a transparent beta-quartz solid solution, or a beta-spodumene solid solution.
Beta-quartz is the hexagonal trapezohedral modification of SiO.sub.2. It exhibits a slightly negative coefficient of thermal expansion (CTE). This makes it of particular interest where thermal cycling occurs, as in cookware. The basis of the beta-quartz solid solution is believed to be the substitution of Al.sup.+3 ions for some of the Si.sup.+4 ions in the beta-quartz structure. The attendant charge deficiency is made up by the introduction of a small ion, such as Li.sup.+, Mg.sup.+2, or Zn.sup.+2, into the beta-quartz structure.
Beta-quartz solid solution glass-ceramics customarily contain TiO.sub.2 as a nucleating agent. Optionally, the TiO.sub.2 may be partially, or wholly, substituted for by ZrO.sub.2 as explained earlier. The appearance of such glass-ceramics can be varied by varying composition and/or heat treatment. Thus, transparent, translucent, or opaque glass-ceramics, which may be water-white, translucent, opaque white, or variously colored, are all possibilities as described in the prior art.
The widest use of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass-ceramic materials has been in the field of culinary ware. For over three decades, Corning Glass Works, now Corning Incorporated, has marketed opaque white cooking utensils under the trademark CORNING WARE. More recently, cooking utensils, formed from a transparent glass-ceramic exhibiting a light brown tint, were introduced commercially by Corning France, S.A. under the the trademark VISION. In general, this transparent glass-ceramic is crystallized at lower temperatures to develop small, beta-quartz solid solution crystals. Such glass-ceramics and their production are described, for example, in U.S. Pat. Nos. 4,018,612 and No. 4,526,872.
It has been observed that transparent, beta-quartz glass-ceramics nucleated with TiO.sub.2 tend to exhibit a light brown tint. This is ascribed to the presence of both TiO.sub.2 and Fe.sub.2 O.sub.3 in the parent glass composition. Efforts have, therefore, been made to either develop a decolorizer or to mask the tint. In an opaque, white glass-ceramic, the brown tint is effectively masked.
Masking, however, becomes more difficult in transparent glass-ceramics. Thus, U.S. Pat. No. 4,093,468 (Boitel et al.) describes problems encountered when attempts were made to mask the brown tint in transparent glass-ceramics. Accordingly, that patent proposed neodymium oxide as a composition additive to make the glass-ceramic sensibly colorless.
The present invention arises from a desire to achieve a decorative color in a transparent, beta-quartz solid solution glass-ceramic. This goes beyond simply neutralizing the inherent brown tint.
In addition to achieving the desired coloration, production demands that it be done while maintaining a certain degree of infra-red transmission in the molten glass. This is necessary to adequately retain heat in the glass during the forming process. Heretofore, this has been accomplished by maintaining the Fe.sub.2 O.sub.3 level in the composition at a small, but reasonably critical, amount.